The config.php file is mostly used for settings from the initial installation, during which they are written to the database that is created and then are no longer read from the file. Some entries, such as the database and database username with password, are still read, so use caution if you feel compelled to edit this file. Modifying the FOG_TFTP_HOST in WebUI->Information->FOG Settings is the correct location, so I’m not sure why it didn’t work for you. You can always try editing the database manually, it’s stored in fog.globalsettings.